This invention relates to a method for fabricating a semiconductor substrate, in particular, to a semiconductor on insulator substrate (SOI) comprising an additional doped layer, in particular an epitaxial layer, suitable for opto electronic applications, such as image sensors.
In optoelectronics, special substrates are necessary which are, for example, used for image sensors, such as backside illuminated CMOS Image Sensors (BCIS), which find their application in video or photographic cameras. In these substrates, photons can be collected by the image sensors formed in the device layer of an SOI substrate. In some devices, the SOI device layer containing the image sensors is transferred to a final substrate to expose the backside of the sensors and facilitate the efficient collection of photons.
In the prior art, this kind of special SOI substrate was prepared using an n-type donor substrate, to form an n-type SOI layer by the conventional SMARTCUT™ layer transfer technology. This method typically comprises the steps of providing a donor substrate, e.g. a silicon wafer, providing an insulating layer on the donor substrate and creating a predetermined splitting area inside the donor substrate. The splitting layer is generally achieved by implanting atomic species or ions, such as helium or hydrogen ions, or both ions, into the donor substrate. In the next step, the donor substrate is bonded to a base substrate, e.g., a further silicon wafer, such that the insulating layer is sandwiched between the handle and the donor substrate. Subsequently, the remainder of the donor substrate is detached from the bonded donor-base substrate at the predetermined splitting area following a thermal and/or mechanical treatment upon the predetermined splitting area. As a result, a semiconductor on insulator (SOI) substrate is obtained.
The use of different dopants in a substrate can lead to cross-contamination of the dopants. For example, the use of a substrate having a first type of dopants (e.g. n-type) in an SOI substrate fabrication line, wherein the substrate has encounters a different substrate containing a second type of dopants (e.g. p-type), can lead to cross-contamination from donor wafers with an impurity dopant concentration of the second type used in the standard SOI substrate, to other wafers having the target dopant type of the first type. Even worse, in the case of special substrates for opto-electronic applications that need a different type of dopants e.g. n-type dopants (phosphorous) compared to standard substrates, e.g., p-type (boron), the n-type dopants may contaminate the fabrication line, thereby reducing the quality of the standard SOI substrates. This thus leads to unsatisfying dopant profiles in both the n-type SOI wafers and the standard p-type SOI wafers.
Surface contamination through airborne contamination is a key concern in this matter. In a standard cleanroom environment with no specific chemical filtering, it is common to have boron or phosphorus contamination on surfaces, in the range of 1 to several 1012 at/cm2 for about 30 min to 2 hours, depending on the air recycling rate. By diffusion, these unwanted elements diffuse into the bulk of a substrate leading to a volume contamination in the of order of 1016 at/cm3, which is a particular problem when n- or p-type layers are targeted.
In addition, during subsequent annealing steps during the typical SMARTCUT™ process, a diffusion of dopants out of the counter-doped layer occurs which further deteriorates the substrate.
Furthermore, special substrates with n-type dopants show a rather high density of defects compared to special substrates with a p-type dopant structure. This is related to the fact that n-type starting substrates, on which an additional epitaxial layer will be grown, have a lower quality than p-type substrates, in particular concerning COP defects.
Thus, there is a need for improvements in these type constructions so that improved BCIS devices can be prepared. The present invention now satisfies this need.